The V8 Owners Forum -

AHH!!
One of those

Cheers

Perry

mgbv8 wrote:What is CVT Dave?

Rubber band gearbox.

I still say it is peak torque gives you peak acceleration in a given gear.
With a CVT, you would run the engine at the constant engine speed which gave peak torque. That would give you max acceleration.

The fly in the ointment for a system with gears is that you have to go past peak torque before shifting because the next gear up will be _below_ peak torque and you have to get back up to it.

Also, a lower gear will give you better acceleration for a given torque because of the torque multiplication of the gear ration. So you want to stay in a lower gear as long as you can (i.e. until redline) as the torque reducing effect of the next gear up will give a corresponding reduction in acceleration. You would only want to shift before the redline if the torque had dropped off to the point where you would get more torque at the wheels by upshifting. i.e. the torque reduction at the redline compared to the target engine speed in the next gear up is more than the gear ratio difference.

An example of how complicated it is is to compare a petrol and diesel car with the same torque. The petrol will out accelerate the diesel because the diesel has to upshift sooner. So in first gear, the petrol will reach 6K rpm, the diesel will only reach 5K rpm. So the petrol gets an extra 1K's worth of acceleration in first. If you increase the torque of the diesel and also it's gearing, so the shift points are at the same engine speed, you could get acceleration parity.

Note I make no mention of BHP. Power is the ability to do work, i.e. push a vehicle along at a constant speed, overcoming drag, friction etc. Torque is a force, which causes a mass to accelerate. This is A-level physics.

Chris.

Think that's spot on, Chris.

DaveEFI wrote:You accurately log your best acceleration times in only one gear on a manual box. Say in 5 mph increments - if you could do such a thing.
Like say 20-25, 45-50 mph, etc.

Where would be best figure be - maximum torque, maximum BHP, or somewhere else?

Perfect surface and no wheel spin. And at the sort of speeds where air drag doesn't make a difference.

get an accelerometer and log it

Although it can be difficult to get nice steady readings. I've plenty of logs from both VBox and my dash, but the raw readings are always very erratic...You can of course smooth them, but the question is how much should you smooth them

With long G data and other info, on the VBox tools software you can then use their formula to give a power graph ( obviously subject to errors, wind resistance, rolling resistance etc If you built more models and did more testing a I'm sure formulas could be derived to account for those too )

But in short..

To calculate BHP enter the following equation
Weight (lbs) * Long Acc (g) * Speed channel (mph) * 0.003054

To calculate kW enter the following equation
Weight (Kg) * Long Acc (g) * Speed channel (kph) * 0.0031107

Problem is, I've that many bloody logs, finding a single gear pull at the minute with no wheelspin etc could take hours lol

For torque graph they use this

To calculate Torque in Nm the user will need to create a new maths channel and enter the following equation.
Power (kW) * 9549 / rotational speed (rpm)

To calculate Torque in ft-lb the user will need to create a new maths channel and enter the following equation.
Power (hp) * 5252 / rotational speed (rpm)

Those interested in where BHP comes from may be interested in the original formula I was taught in Applied Mechanics at school, many years ago.

It was remembered as 'Two pies and tea'.

So 2 x pi x rpm x torque(lb.ft)
-------------------------------- = BHP
33,000

33,000 ft.lb per minute being 1HP.

This is more conveniently condensed into

RPM xT
-------
5252